A wide variety of implantable medical systems that deliver a therapy or monitor a physiologic condition of a patient have been clinically implanted or proposed for clinical implantation in patients. An example implantable medical system may include an implantable medical lead connected to an implantable medical device (IMD). For example, implantable leads are commonly connected to implantable pacemakers, defibrillators, cardioverters, or the like, to form an implantable cardiac system that provides electrical stimulation to the heart or sensing of electrical activity of the heart. The electrical stimulation pulses can be delivered to the heart and the sensed electrical signals can be sensed by electrodes disposed on the leads, e.g., typically near distal ends of the leads. Implantable leads are also used in neurological devices, muscular stimulation therapy, gastric system stimulators and other implantable medical devices (IMDs).
Patients that have implantable medical systems may benefit, or even require, various medical imaging procedures to obtain images of internal structures of the patient. One common medical imaging procedure is magnetic resonance imaging (MRI). MRI procedures may generate higher resolution and/or better contrast images (particularly of soft tissues) than other medical imaging techniques. MRI procedures also generate these images without delivering ionizing radiation to the body of the patient, and, as a result, MRI procedures may be repeated without exposing the patient to such radiation.
During an MRI procedure, the patient or a particular part of the patient's body is positioned within an MRI device. The MRI device generates a variety of magnetic and electromagnetic fields to obtain the images of the patient, including a static magnetic field, gradient magnetic fields, and radio frequency (RF) fields. The static magnetic field may be generated by a primary magnet within the MRI device and may be present prior to initiation of the MRI procedure. The gradient magnetic fields may be generated by electromagnets of the MRI device and may be present during the MRI procedure. The magnetic and RF fields may be generated by transmitting/receiving coils of the MRI device and may also be present during the MRI procedure.
If the patient undergoing the MRI procedure has an implantable medical system, the various fields produced by the MRI device may have an effect on the operation of the medical leads and/or the IMD to which the leads are coupled. For example, the gradient magnetic fields or the RF fields generated during the MRI procedure may induce energy on the implantable leads (e.g., in the form of a current). The current induced on the implantable leads may cause the IMD to sense a cardiac signal when one is not present, a phenomenon referred to as oversensing, or to not sense a cardiac signal when one is present, a phenomena referred to as undersensing. Oversensing and undersensing may result in the IMD delivering therapy when it is not desired or withholding therapy when it is desired. A need exists for improvements to IMDs that reduce or eliminate the impact of magnetic fields to the IMD operation.